The apparatus of the present invention will hereinafter be described with particular reference to wind propelled craft being sailing vessels such as sailing dinghies, sailing catamarans, or sailing keel boats. However, it is understood that the apparatus is of general applicability.
In general sails used to propel craft are either relatively thin, compared to their length, or comprise combinations of thick symmetric aerofoils, such as in AC72 class catamarans seen in the 2013 America's Cup competition.
Sails or aerofoils create lift by the action of a differing air velocity from one side of the sail to the other. Stagnation of the air at the luff or mast on the windward side of the sail, along with the shape of the aerofoil section, creates an asymmetry in the air path from the windward to the leeward sides of the sail or aerofoil. Consequently, given that other aerodynamic conditions are satisfied regarding the joining of the flow streams downwind of the sail or aerofoil, this asymmetry creates increases the speed of the air on the leeward side relative to the windward side and hence an pressure difference between the leeward and windward sides. This pressure differential results in aerodynamic lift. It is this lifting force which then propels the vessel. At the same time, the sail or aerofoil also produces aerodynamic drag, which when sailing upwind, for example, can reduce the force which propels the vessel. Hence it is desirable for wind propelled vessels to be able to produce relatively high lift and relatively low drag, particularly for sailing with the wind forward of the beam of the vessel.
While the thick symmetrical wing sails used on AC72 catamarans and other high performance sailing craft can provide good lift and drag characteristics. These wings are typically made from two or more symmetrical wing sections configured relative to one another to provide camber. There is no ability to induce asymmetry into the individual wing sections. Thus, these wing sails have practical limitations. These wing sails are constructed to be light weight and rigid. They cannot be collapsed tor easy storage.
Further, they cannot be reefed or stowed when sailing if it is desirable to have less sail area. These wing sails are also generally quite fragile and can be easily damaged in the event of a capsize or collision.
While reefable and stowable “soft” wing sails exist, they are genrally quite complex and heavy.
A wing sail is described by Johnston, Patrick Murray (in patents AU1986052399 and U.S. Pat. No. 4,766,831, the contents of which are incorporated herein by reference). The wing sail comprises two substantially identical flexible sail portions each having a leach and a luff. Elongated battens are in contact with the flexible sail portions. A control rod has the battens and luffs of the sail portions rotatably attached so that the perpendicular distance between the sail portions varies wherein angular displacement of the control rod with respect to the boom causes the battens to be compressed along their length so as to bend one of the sail portions to increase the camber thereof and causes the battens of the other sail portions to be tensioned along their length so as to partly straighten the other sail portion which decrease the distance from each other at the luff, but increase the perpendicular distance between the sail portions away from the luff, resulting in asymmetry of the aerofoil. Notably the leaches of the sail portions are connected. Further the control rod is recessed in a concave shaped trailing edge of a mast. A leading edge of the mast is semi-elliptical and aerodynamic, so as to have a wing mast profile.
The present invention provides improvements over this prior wing sail.